Try squeezing the handles really hard. When I fist got my JST crimps I could not get the jaws open either. Turns out they just take alot of pressure to get to the last ratchet position to click, then they popped right open.
For production use the action is binary by design. Either they successfully complete the join, or they destroy the crimp pin. They often don’t have a manual release.
to release the jaws after a “dry-fire” like this one. complete the action by squeezing the jaws fully closed which will allow them to release.
These ones look like PIDG (or equivalent) 14-16AWG so completing the action by squeezing the jaws fully closed may require some force